Did Horses Act as Intermediate Hosts That Facilitated the Emergence of 1918 Pandemic Influenza?

Horses may have served as intermediate hosts that helped spread an avian-origin influenza virus, contributing to the emergence of the 1918 pandemic influenza. This theory is supported by a severe equine influenza outbreak during World War I, which allowed widespread horse movement and potential viral mixing.

Background and Hypothesis

• The 1918 influenza pandemic’s ecological origins are not fully understood.
• Researchers propose that horses could have played a key role as intermediate hosts.
• Specifically, a prepandemic avian-origin influenza virus may have circulated in horses prior to 1918.

Evidence Supporting the Role of Horses

• Historical reports documented a severe epizootic (animal outbreak) of equine influenza starting in 1915, marked as unusually severe compared to typical outbreaks.
• This severe equine influenza epidemic coincides with the period before the 1918 human pandemic, suggesting horses were infected with a significant influenza strain.

Impact of World War I on Equine Influenza Dynamics

• World War I created extremely high demand for horses, especially in North America, between 1914 and 1918.
• This demand led to one of the largest movements of horses in history, involving transportation across regions and close contact among large groups of horses.
• Such extensive movement and mixing among horses offered numerous opportunities for influenza viruses to reassort — to exchange gene segments — between avian-origin and human influenza viruses infecting horses.
• These reassortments could have generated new influenza viruses capable of infecting humans, potentially triggering the 1918 pandemic.

Future Research Directions

• To test the hypothesis, there is a need for analysis of archived equine tissue samples or serum collected during or before the 1918 pandemic period.
• Detecting and characterizing influenza viruses in these historical samples could confirm whether horses carried avian-origin or novel reassorted viruses leading up to 1918.
• This research could provide valuable insight into the ecological and evolutionary pathways that led to the emergence of one of the deadliest influenza pandemics in human history.